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Mo-USY zeolites for hydrodesulphurization. I.
37
Applied Catalysis A: General, 99 (1993) 37-54 Elsevier Science Publishers APCAT
B.V., Amsterdam
A2507
MO-USY zeolites for hydrodesulphurization. I. Structure and distribution of molybdenum oxide phase J.A. Anderson, B. Pawelec and J.L.G. Fierro Znstituto de Catcilisisy Petroleoquimica, C.S.Z.C., Campus UAM, Cantoblanco, 28049 Madrid (Spain) (Received 19 January 1993)
Abstract Ultrastable (USHY) zeolite has been used as a support material for a series of molybdenum catalysts prepared using various precursors and activation procedures. Studies using atomic absorption spectroscopy, X-ray photoelectron spectroscopy, Fourier transform IRof the mid-infrared range and of adsorbed nitric oxide and pyridine, and sorption capacity of water indicate that the distribution of molybdena in the zeolite is strongly influenced by the preparation procedures. Catalysts calcined by a non-conventional procedure, involving decomposition of the precursor by heating under vacuum over extended periods of time, show a migration of the molybdena from the external surface into the lattice cavities. Keywords:
catalyst characterization
(FT-IR,
XPS, XRD);
molybdena-USY
zeolites
INTRODUCTION
Supported molybdenum-based catalysts represent an important group of catalysts used in hydrotreating processes. The structure of the active phase in such catalysts is of paramount importance in determining catalytic activity. In this respect, the method of active phase incorporation and the nature of the support have been shown to play an important role in the kind of molybdena phase produced using conventional supports [ 11. In the case of zeolite supports with uniform pores in the framework structure, one may expect to form molybdenum species which are higher dispersed and with a more clearly defined structure than those species obtained over amorphous silica or alumina supports. Amongst these zeolites, ultra&able Y is of considerable interest due to its high thermal and chemical stability. However, in addition to the low ionexchange capacity of USY zeolite, conventional ion-exchange procedures are inapplicable in the case of molybdenum due to the lack of simple cationic forms Correspondence to: Dr. J.A. Anderson, Instituto de Cat&lisis y Petroleoquimica, UAM, Cantoblanco,
0926*860X/93/$06.00
28049 Madrid, Spain. Tel. (+34-1)5854773,
0 1993 Elsevier Science Publishers
B.V.
C.S.I.C., Campus
fax. (+34-1)5854760.
All rights reserved.
38
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54
of molybdenum which exist under conditions in which exchange would be possible. Various preparation methods for molybdenum loaded zeolites have been described and the resultant materials characterised in the forms MoHY, MoNaY and MOUSY. These methods include solid-state ion exchange using MoCl, [ 241, adsorption-decomposition of MO (CO), [ 5-71, and conventional impregnation with aqueous solutions of ammonium heptamolybdate (AHM) [8-lo]. Although the latter impregnation method produces essentially an external surface loading of molybdenum due to the failure of oxoanionic or neutral complexes to penetrate the zeolite cavities in the presence of water, thermal decomposition of these species under constant low vapour pressures of water produces a redispersion of molybdenum due to solid-state exchange of MoO,,(OH), within the zeolite pores [ 11,121. This procedure has been successfully applied in the cases of NaY and NH,NaY zeolite catalysts [ 11,121. In this paper, characterization data are reported for a series of MOUSY catalysts in which molybdenum has been incorporated using the above procedures. In the accompanying paper [ 131, characterization data for the sulphided catalysts along with activity and selectivity data for the hydrodesulphurisation of thiophene will be presented. EXPERIMENTAL
Catalyst preparation Four molybdena-loaded ultrastable Y (USY) zeolite catalysts were prepared using various procedures and different precursors. All the catalysts were prepared from the original ultrastable HY zeolite, kindly supplied by CONTECA, B.V., Surte (Sweden). Zeolite characteristics were: (a) USY (Conteca); Si0.J A1203 mole ratio 5.6, Na,O content 0.14 wt.-%, unit cell 2.454 nm and (b) NaY (LZ-Y52 Union Carbide); SiO,/Al,O, mole ratio 4.8, Na,O/Al,O, - 0.97. Catalyst MO-Cl was prepared by solid-solid ion exchange by mixing the starting USY zeolite with MoCl, [2-41. The USHY zeolite was firstly dehydrated in flowing helium at 493 K for 2 h to minimise hydrolysis of MoCl, by molecular water held by the zeolite. The dried zeolite was immediately mixed with anhydrous MoCl, and ground under air using a mortar and pestle. The resulting mixture was subsequently introduced in a flow reactor and heated in flowing helium at 673 K for 6 h. After this pretreatment, the catalyst was dried at 383 K and calcined at 773 K in air. Catalyst MO-CO was prepared by wet impregnation of USHY zeolite with the neutral MO ( CO)6 complex [5-71. This impregnation was carried out with a Mo(CO),-benzene solution whose volume was selected to achieve a final MOO, content of 6.0 wt.-%. The adsorbed complex was decomposed under vac-
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54
39
uum at 333 K and then dried in air at 383 K for 1 h. Calcination was conducted at 723 K for 1 h in a glass vessel. Two catalysts were prepared by conventional aqueous impregnation of an USY zeolite with ( NH1)6M07024-4H20 (AHM) [ 8-10 1, followed by removal of water in a rotary evaporator. These catalysts will be referred to as 4Mo-A and 8Mo-A, indicating low and high molybdenum contents, respectively. The impregnates were dried in vacuum for 8 h and later in air at 383 K. Calcination was achieved at 773 K at a low and constant pressure of water (4. 10m2Torr, 1 Torr= 133.3 Pa) and extended over a period of time of 800 h [ 11,121. For comparative purpose an MoOJNaY zeolite catalyst (MoNa-Cl) was prepared according to the same procedure used for the MO-Cl sample.
Catalyst churacterization Atomic absorption spectroscopy (AAS) Molybdenum contents were determined using a Perkin-Elmer 3030 atomic absorption spectrometer. Samples were prepared for analysis by dissolving in a mixture of HF, HCl and HNO, before heating in a microwave oven at a maximum power of 650 W.
Adsorption capacities Adsorption of water in the molybdenum-loading zeolites was measured using a Cahn-2000 vacuum microbalance. All samples were thoroughly outgassed at 623 K and were contacted with the adsorbate at 298 K at relative pressures (Pl%) between 0 and 1. The curves were constructed in graduated steps by increasing the pressure in small successive increments by evaporating distilled water from an ampoule. The equilibrium pressure was measured with a baratron MKS capacitance pressure transducer connected to the same vacuum line. From the isotherms of water adsorption, the maximum volume of the zeolites accessible to water ( IV,,) was calculated from the ordinate at the origin of the line in the linear form logW= f [ (TlogP/P,,)‘] as defined by the Dubinin-Radusehkevitch model. Nitrogen adsorption isotherms were measured at 78 K using a Micromerics Digisorb 2600 on samples which were previously outgassed at 623 K.
X-ray diffraction (XBD) X-ray powder diffractograms of the catalysts were recorded on a Philips PW-1140 diffractometer using Cu Kcr radiation and a nickel filter. Power diffractograms of MO-USY and MoNaY zeolites samples were recorded over a range of 28 values from 5 to 40’.
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54
Infrared spectroscopy Mid-infrared spectra in the range 1200-400 cm-’ were obtained by pressing wafers of 1% MO zeolite in KBr. For FT-IR studies of adsorbed nitric oxide, catalysts in the form of self-supporting wafers of about lo-12 mg/cm2 thickness were reduced in a hydrogen flow at 773 K for 2 h in a portable infrared cell equipped with greaseless stopcocks and KBr windows. After reduction the catalysts were outgassed in dynamic vacuum for 0.5 h at the same temperature and then cooled to room temperature. Catalysts were subsequently exposed to 30 Torr NO and the IR spectra of chemisorbed nitric oxide recorded using a Nicolet ZDX spectrophotometer at a resolution of 4 cm-‘. Spectra of adsorbed nitric oxide were obtained by subtraction of a background spectrum prior to nitric oxide exposure in order to eliminate an overtone band of the zeolite lattice which appears in this region. X-ray photoelectron spectroscopy (XPS) X-ray measurements were performed with a commercial XPS spectrometer (Fisons escalab 200R) equipped with a Mg Ka X-ray source (h y = 1253.6 eV) and a hemispherical electron analyzer. The X-ray source was operated at 12 kV and 10 mA. The samples were pressed into small stainless steel cylinders and mounted onto a manipulator which allowed the transfer from the preparation chamber into the spectrometer. The samples were pumped out to 10M5 Torr before they were moved into the analysis chamber. The residual pressure in this ion-pumped chamber was maintained below 7*10-’ Torr during data acquisition. Each spectral region of the photoelectrons of interest was scanned several times to obtain good signal-to-noise ratios. The Cls peak at a binding energy of 284.9 eV was taken as an internal standard. RESULTS
AAS The chemical composition of the calcined MO-USY zeolites is presented in Table 1.The extent of molybdenum-exchange was rather low for catalyst MoCO, considering that the volume of solution of MO ( CO)6 was chosen to give a MOO, content of 6.0 wt.-%. For catalysts 4 and BMo-A, prepared by conventional aqueous impregnation of an USY zeolite with AHM and calcined by an isobaric thermal decomposition procedure, MOO, contents obtained were close to those calculated theoretically.
41
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54 TABLE 1 Chemical composition, infrared and XPS data for Mo-zeolite catalysts Catalyst
wt.-%
1730+ 1820 (cm
8Mo-A MO-Cl MoNa-Cl 4Mo-A MO-CO
8.6 6.3 5.4 3.9 3.1
-1
10.62 1.53 1.12 3.08 1.37
10
9201460 cm-‘/cm-’
MO 3dS,zb (eV)
0.162 0.104 0.006 0.081 0.059
232.3 232.4 232.3 232.8 232.6
IMcJzSi
0.526 0.446 0.149 0.174 0.462
“Integrated absorbance measured by nitric oxide adsorption at room temperature on Hz-reduced catalysts. batheaccuracy of each measurement is within 0.2 eV. ‘Intensity areas of the MO 3d and Si 2p peaks normalized for the number of scans.
,
20
35
28 angle (“) Fig. 1. XRD patterns for (a) USY, (b) 4Mo-A, (c) MO-CO, (d) MO-Cl, and (e) 8Mo-A.
X-ray diffraction The diffraction patterns of the catalysts and the USY zeolite (Fig. 1)showed peaks at d-spacings of 0.5644, 0.4733, 0.4348, 0.3750, 0.3285 and 0.2839 nm
42
J.A. Anderson
et al./Appl. Catal. A 99 (1993) 37-54
which are typical of USY zeolites. The crystallinity changes were estimated for all zeolite catalysts by comparing the three peaks at 0.5644, 0.3750 and 0.2839 nm with those of the parent USY zeolite. A slight decrease of ca. 10% was observed which can be explained in terms of the dilution effect of the molybdenum oxide added to the base zeolite [ 51. These results differ markedly from MoNa-Y catalysts which show larger crystallinity losses produced by calcination treatments [ 121. The MOO, phase, whose principal reflection for the orthorhombic phase occurs at a d-spacing of 0.345 nm (ASTM 21-569), was observed only for 8Mo-A and MO-CO zeolites, despite the fact that these samples contained the highest and lowest molybdenum contents, respectively (Table 1). Infrared spectroscopy Mid IR range, 1200-400 cm-’ Spectra of molybdenum loaded USY zeolites in KBr exhibited bands at 1200, 1070, 835,605, 503 and 465 cm -’ due to framework vibrations of the zeolite [4,7,14]. The presence of these bands in spectra for all the catalysts and the absence of significant intensity losses confirms that the crystallinity was retained for calcination temperatures between 723 and 773 K. Consistent with previous studies of MoY zeolites [ 4,6,7,14], an additional maximum at ca. 900 cm-l, not present for the USY zeolite in the absence of molybdenum, was detected for all samples. MO-O vibrations may be expected to give IR active bands in the range 800-1000 cm-’ [ 61. If this band is indicative of Moos, then this phase must have been present for all samples despite the detection by XRD (Fig. 1) in only 8Mo-A and MO-CO samples since the band at ca. 900 cm-’ was present for all catalysts. The integrated absorbance of the IR band at ca. 900 cm-’ (as a ratio using the structure insensitive band at 465 cm-’ as an internal standard) is shown in Table 1 for the molybdenum-loaded zeolite catalysts. On comparing these values with the Moos contents, it is clear that an increase in loading does not give an increase in intensity despite several reports to the contrary [4,14 1. The anomaly of sample MO-CO, which has a crystalline MoOB phase detected by XRD although having the lowest molybdenum content and the second lowest band intensity at ca 900 cm-’ (Table 1 ), would indicate that the band at ca. 900 cm-l is due to some surface MO-O species. This is consistent with the perturbation of a similar maximum in the presence of ammonia [ 71. It is of interest that the band maximum appears at ca. 895 cm-’ for samples MO-Cl and MO-CO but at ca. 920 cm-l with an unresolved lower frequency maximum for samples MO-A. Abdo and Howe [ 71 report the presence of resolved maxima at 900, 878 and 850 cm-’ following activation at 773 K of Mo(CO), in Y zeolite. Previous reports indicate that the frequency of the Mo=O vibration in this region depends on the coordination number and the number of terminal oxygen atoms on the molybde-
J.A. Anderson et al. jApp1. Catal. A 99 (1993) 37-54
43
num (VI) [ 151. Alternatively, a decrease in frequency in the Mo=O band may implicate an increase in the oxidation state of molybdenum [ 161. Fig. 2 shows the dependence of the intensity of the combined band envelope at ca. 900 cm-’ on the time of calcination under vacuum for catalysts 4Mo-A and 8Mo-A. A progressive increase in this intensity occurs for sample 4Mo-A during the course of the calcination treatment, whereas for 8Mo-A the growth is initially more dramatic but declines after extended periods at 773 K. Nitric oxide adsorption Two absorption bands at ca. 1730 and 1820 cm-’ were observed for nitric oxide adsorbed on all reduced samples as shown in Fig. 3A and B for 4Mo-A and 8Mo-A examples. These bands may be attributed to paired nitric oxide molecules held as dinitrosyls adsorbed at partially reduced, probably Mo4+ sites [ 17,181. This doublet appears at higher wavenumbers than for conventional molybdena-alumina catalysts [ 181, as in the case of dinitrosyl species formed on other metal oxide-loaded Y zeolites. An indication of the relative proportion of coo&natively unsaturated (CUS) reduced molybdenum sites for each catalyst was obtained by measuring the total integrated absorbance at ca. 1730 and 1820 cm- ’ (Table 1). The order of molybdena exposure in the various catalysts follows the trend: 8Mo-A > 4MoA > MO-Cl > MO-CO > MoNa-Cl. Spectra obtained for reduced samples of 8MoA and 4Mo-A as a function of calcination time at 773 K are shown in Fig. 3
3,5-
,c
-0906
/ / 3,0-
-0,07
2,6-
-0,06'2
z is 0 -0,os
w-
- 630
l,6-/
fl
-0,04
B 1,0I0,03 10 0
Calcination
time
(days)
Calcinatlon
20
time
30
40
(days)
Fig. 2. Dependence of the absorbance ratio 9001465 cm-’ and on the combined integrated intensities of nitric oxide absorption bands at 1820 and 1730 cm-’ on the calcination time for (A) 8Mo-A and (B) 4Mo-A.
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54
44
1860
1760
1660
1860
Wavenumber
(cm-‘)
1760
1660
Fig. 3. IR spectra of adsorbed nitric oxide at room temperature on H,-prereduced MO-loaded zeolites: (A) 4Mo-A; (B) 8Mo-A for calcination times of (a) 1, (b) 5, (c) 15, (d) 23 and (e) 33 days. TABLE 2 IR and XPS data for 4Mo-A catalyst calcined under vacuum for different periods of time
t
MO3d.w
(days)
(ev)
1 5 15 23 33
232.7 232.6 232.8 232.7 232.8
blol&.P
A
1730 -1
(cm
0.200 0.153 0.158 0.190 0.174
)
b
0.54 1.28 1.56 1.61 1.74
DSurfaceatomic ratio determined from XPS spectra. bInt.ergatedabsorbance measured by nitric oxide adsorption at room temperature on Hz-reduced catalysts.
with the integrated band intensities collected in Tables 2 and 3. In Fig. 2, the changes in the combined intensities at 1730 and 1820 cm-’ as a function of calcination time may be compared with the change in intensity of the band at ca 900 cm-’ due to MO-O vibrations. Sample 8Mo-A displays a decrease in the
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54 TABLE 3 IR and XPS data for SMo-A catalyst calcined under vacuum for different periods of time
t
MO W/z (days) (eV)
h0lh.i”
A 1730 cm-l’
1 5 15 23 33
0.689 0.447 0.570 0.442 0.526
1.77 5.35 6.90 11.34 6.5
232.4 232.4 232.3 232.6 232.3
“Surface atomic ratio determined from XPS spectra. bIntergated absorbance measured by nitric oxide adsorption at room temperature on Hz-reduced catalysts.
total absorption after extended periods of time whereas the increase for sample 4Mo-A is less dramatic but continual. In addition to changes in the overall intensities as a function of calcination time, spectra for sample 8Mo-A clearly show changes in the relative intensities of bands due to asymmetric and symmetric stretching vibrations and a shift to lower frequencies for the latter maximum, which are not displayed by the 4Mo-A sample. This shift has been interpreted as indicative of an increase in the degree of unsaturation [ 191. Pyridine adsorption As hydroxyl groups, either as a source of protons or in condensation processes, react with MoCl, (or MoOCl,) [ 2-41, MO ( CO)6 [ 5-71 or with molybdate anions derived from the calcined precursors [ 121, experiments were carried out involving the adsorption of pyridine to determine the extent to which proton acidity was suppressed following incorporation of molybdenum. Brensted acid sites are typically identified by a band at ca. 1540 cm-’ corresponding to pyridinium ions and Lewis acid sites by a band near 1440 cm-’ due to coordinated pyridine molecules [ 201. From the spectra in Fig. 4 obtained by adsorption of pyridine at 298 K followed by outgassing at 393 K, an estimation of the ratio of Brensted to Lewis acidity was calculated from the ratio of integrated band intensities at ca. 1540 and ca. 1440 cm-‘. In decreasing order these were: USY > MO-CO > MO-Cl z 4Mo-A x 8Mo-A > MoNa-Cl. The detection of Brensted acidity for the Na-Y zeolite (Fig. 4) is surprising although sodium doped alumina samples containing an excess of these cations also exhibit proton acidity after calcination at 573 K [ 201. The MoNa-Cl sample exhibited a band at 1444 cm- ’ due to adsorbed pyridine which was detected at 1452 cm-’ for the MO-USY samples indicating weaker Lewis acid centres in the presence of sodium. This is consistent with the shift to lower frequencies disfilayed by pyridine adsorbed on sodium doped A1203 compared with sodium free A1203 [ 201.
46
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54
1700
1600
1500
1400
Wavenumber(cm-‘) Fig. 4. IR spectra of pyridine adsorbed at 298 K on (a) MoNa-Cl, (b) USY, (c) MO-Cl, (d) MOCO, (e) 4Mo-A, (f) 8Mo-A.
X-ray photoelectron spectra XP spectra for the calcined MO-USY zeolites and MoNa-Cl catalyst are shown in Fig. 5 and the Mo3d binding energies summarised in Table 1. The binding energy of the most intense Mo3d peak at ca. 232.5 eV was essentially the same for all catalysts and is consistent with values expected for fully oxidized molybdenum [ 4,111. The Mo3d peaks for the MoNa-Cl sample (Fig. 5e) shows a third component towards lower binding energies. As additional peaks with the same shifts were observed for A12p, Si2p and Nals using the same sample, this may be attributed to charging effects. The Mo3d peak intensities relative to silicon are given in Table 1 for the series of samples while the surface atomic ratios of Mo/Si for the 4Mo-A and 8Mo-A catalysts calcined under vacuum for different periods of time are listed in Tables 2 and 3, respectively. Both samples show lower Mo/Si ratios after prolonged calcination periods compared to values after 1 day calcination. Fig. 6 shows the relationship between XPS Mo/Si ratios and the overall (1730 and 1820 cm-‘) absorbance for the series of Mo-USY zeolites and MoNaY sample. Points for MO-Cl, MoNa-Cl and MO-CO lie on an straight line show-
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54 MO 3d
47
512
1
I
I
240
235
230
BE (eV) Fig. 5. Mo3d core level spectra for (a) 4Mo-A, (b) 8Mo-A, (c) MO-Cl, (d) MO-CO and (e) MoNaCl.
ing a correlation between the two factors [ 191. However, the MO-A samples, which adsorbed higher quantities of nitric oxide do not show the same correlation, with sample 8Mo-A in particular, far from the line indicating a considerable fraction of molybdenum was accessible to nitric oxide but not detected by KPS. Water acEsorption Water adsorption isotherms at 298 K for the various molybdenum-loaded zeolkes and the parent USY zeolite show a rapid increase in water adsorption at P/P0 below 0.2, followed by a slow increase up to P/P,, of 0.9. As these
48
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54
2
Absorbance
(au x cm.‘)
Fig. 6. Relationship between the XPS Mo/Si ratios and the overall (1820 + 1730 cm-’ ) integrated absorbance of bands due to nitric oxide for 4Mo-A (0 ), MoNa-Cl ( A ), MO-Cl (m), MO-CO ( •I1 and 8Mo-A xeolites ( 0 ) . TABLE 4 Water adsorption at 298 K (P/P,,=O.4) USY zeolites
and nitrogen adsorption at 78 K (P/P0=0.2)
Zeolite
W wetel (w/g,,)
Woo (cm3/h)
vNitmgen
4Mo-A 8Mo-A MO-Cl MO-CO MoNa-Cl USY NaY
238 205.7 245.5 315 213.3 355 331.8
0.257 0.219 0.256 0.331 0.211 0.372 0.331
125.6 124.9 133.0 155.9 120.5 134.6 190.0
for Mo-
(cm”/g,,J
“W, is the maximum volume of the zeolite accessible to water.
isotherms indicate the amount of liquid water condensed in the zeolites pores and cavities, a comparison of the extent of water adsorbed can be made at a fixed P/P0 value of 0.4 (Table 4). From these results of water sorption capacity, an indication of the location of Moo3 in the different molybdenum-loaded zeolites may be obtained. Similar values of sorption capacity were obtained for the MO-CO zeolite and unloaded USY and Nay. The slight decrease for MoCO may be a diluent effect of MOO,. The high sorption capacity of the MO-CO sample is consistent with the external location of a poorly dispersed MOO, phase in the calcined sample. The differences are much more notable for the other samples. The 4Mo-A, MO-Cl and 8Mo-A samples gave rather low values. As both 4Mo-A and 8Mo-A zeolites were prepared by impregnation with heptamolybdate anions which cannot enter the zeolite pores, the micropore vol-
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54
49
ume, and water sorption capacity, would be expected to be similar to the parent USY zeolite. As indicated in previous studies [ 2,111, nitrogen isotherms for zeolites cannot be fitted to the BET equation, showing instead type 1 behaviour. However adsorption capacities at P/P,, = 0.2 can be compared with the unloaded zeolite to yield information regarding the internal location of molybdena species [ 21. The results for nitrogen adsorption (Table 4) are in general agreement with the data for water adsorption with samples 8Mo-A, 4Mo-A and MO-Cl showing the highest losses in volume compared with the unloaded zeolite, and MO-CO showing a relatively smaller loss. The maximum loss in capacity of 32% (~MoA) is identical to a value reported for 6% MO in mordenite after activation at 723~K [2]. DISCUSSION
The combination of mid infrared and XRD measurements indicates that the crystallinity of the MOUSY zeolites is maintained even when the precursors are activated at temperatures of 723 to 773 K, unlike MO-Y zeolite where appreciable losses in crystallinity occur in this temperature range [ 4,7,12]. From Fig. 6, where the Mo/Si ratio from XPS has been plotted against the integrated intensity of the IR bands of adsorbed nitric oxide, the MO-A samples and in particular 8Mo-A appear anomalous in the series. The values for the other samples lie close to a straight line indicating correlation between the two aforementioned parameters and consistent with similar measurements for Mo/A1203 [ 191. The considerable deviation from the line of data for BMo-A, and to a lesser extent for 4Mo-A, would suggest the presence of a molybdenum phase, accessible to nitric oxide molecules, but located within the inner zeolite structure and hence not detected by XPS. This does not mean that the other samples contained only externally located molybdenum phases: only that the rel: at&e proportion was higher for the MO-A samples. The extent of molybdenum location within the internal zeolite structure may be obtained from the occlusion of water and nitrogen adsorption relative to the molybdenum free USY zeohte (Table 4). These data clearly indicate the relatively large volumes in the :inner zeolite structure penetrated by molybdenum phases for 8Mo-A and MoNa-Cl. As 8Mo-A and 4Mo-A were prepared using heptamolybdate anions which are not expected to enter the zeolite pores in the presence of water [8121 this suggests a transfer of molybdenum occurs during the long calcination procedure adopted. This would be consistent with the variation in the number of nitric oxide adsorption centres (Tables 2, 3, Fig. 2) as a function of the number of days of calcination and also the drop in Mo/Si ratio following the first day of calcination. The redispersion mechanism of molybdena in zeolites has received some attention [11,12,14], and leads to the migration of MOO, from the external
50
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54
zeolite surface into the lattice cavities during calcination. Such a process may involve vaporization of MOO, and/or MOO, (OH) 2, the latter being formed according to the equilibrium: MoO,(s)+H,O(v)~MoO,(OH),(v)
(1)
Although the vapour pressure of MOO, and Moz (OH), are low at 773 K [lo], the extended calcination periods and the short distances (the same order as the zeolite grains) across which the diffusion process needs to be operative, allows an efficient molybdenum redistribution. The molybdena concentration will tend towards uniformity throughout the zeolite structure, except inside the smaller cavities where steric hindrance effects may be important. Assuming a diameter of 0.47 nm (equivalent to 0.17 nm2 cross section) for adsorbed Moos or Moo2 (OH),, entrance into the sodalite units and hexagonal prisms would be diffusion controlled at high temperatures. The resultant molybdenum species would be highly dispersed and located on a large variety of sites producing a wide heterogeneity in molybdenum-zeolite interactions. The variation in the intensity ratio of the bands due to the asymmetric/symmetric stretching vibrations of adsorbed nitric oxide indicating a change in dipole angle, has been related to the different topological characteristics of the surface structure of the molybdenum phases [ 211. Both samples after 1 day calcination show nitric oxide adsorbed in a form with a dipole angle (Q) of ca. 69”. However whereas the value for 4Mo-A remains constant thereafter at ca. 100” the values for 8Mo-A vary from 100 to 132”, indicating a continual change in the nature of the CUS molybdenum sites. A redispersion of molybdenum species for 8 and 4Mo-A samples, would be consistent with the changes in band intensity at ca. 900 cm-’ as a function of calcination time (Fig. 2). The 8Mo-A and 4Mo-A samples also exhibited initial increases in the quantity of CUS reduced MO*+ sites as monitored by the intensity of the vN0 band (Fig. 2), indicating a possible connection between these measurements. This apparent correlation need not indicate that these parameters (reduced CUS nitric oxide adsorption sites and band intensity at ca. 900 cm-‘) are related to the same molybdenum species. This is evident from the different numerical order of magnitude for the two parameters given in Table 1 for the complete series of samples. A redispersion (increase in intensity at ca. 900 cm-‘) may produce adsorbed molybdenum species, stable to reduction in hydrogen at 773 K, and therefore incapable of nitric oxide adsorption. Such a species would be consistent with the non-reducible tetrahedral molybdenum species formed at low loadings on A1203 [ 221. In this respect, fully calcined 8 and 4Mo-A samples exhibited an intense maximum at ca. 230 nm in the diffuse reflectance UV spectra, consistent with the presence of molybdenum in tetrahedral coordination [lo] with only a very weak maximum at 330 nm for the 8 MO-A sample [ 231. The presence of the latter maximum, characteristic of Moo3 [lo], is consistent with the detection of this phase by
51
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54
XRD (Fig. le). The formation of this tetrahedral species is consistent with the growth in band intensity at ca. 900 cm-‘, the development of the higher frequency maximum at 920 cm-’ for MO-A samples, and with previous assignments to MOO,‘- species in MoUSHY, MoNaY and MoY zeolites [4,10,24]. The higher frequency maximum compares with a band at 920 cm-’ observed at low molybdenum loadings on SiO, and attributed to pseudomolybdates [ 251. Daiiand Lunsford [ 41 attributed a similar band to species (I) in MOUSY zeolite,;whereas the development of the higher frequency band leads us to believe the growth in this region as a function of time (Fig. 2) to be due to formation of species (II), where z indicates an oxygen ion in the zeolite lattice. OH
o3\ 03
/
MoVI< 03
0 o\\ MoVI< oz OZ’
(11)
(1)
The tetrahedral cis-dioxo-molybdenum (VI) complexes have Mo=O frequencies of 876-913 and 909-943 cm-’ [26]. The formation of such species may be readily envisaged, following reaction of the mobile species produced in eqn. (1) with the surface.
MoO,(OH),
+ 2(OH)z
-
O\\ //O
+ 2H,O
(2)
oz~Mo~oz Despite the increase in quantity of these species formed as a function of Cal&nation time (Fig. 2), the unreducible nature of these MoV’ species in hydrogen at 773 K [ 10,171 makes these centres incapable of nitric oxide adsorption. The adsorption sites for nitric oxide are probably located on the larger polymolybdate structures or MOO, crystallites. Break up of these with the formation of mobile MoOz (OH )Z species would initially create additional exposed reducible MoV’ ions and subsequently an increase in the number of nitric oxide adsorption sites. The above conclusions are in line with the speculative proposal of Jung et al. [27], that only MOO, species, such as bulk MOO, crystalliteg, which are not directly bonded to the support may chemisorb nitric oxide. In a previous study [ 111, it was suggested that the bonding of the mobile MoOz (OH), type species to MOO, was weaker than to the zeolite surface so that after an efficient redispersion on the external surface, a higher temperature was required to detach these species and allow further diffusion into the bulk of the solid and the zeolite small cavities. The loss in nitric oxide adsorp-
52
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54
tion capacity (Fig. 2)) may be explained in a similar manner: not so much following an increase in temperature but due to the extended period of time allowing the mobile species to enter cavities inaccessible to nitric oxide. Abdo and Howe [ 71 report that for MoHY, an increase in the extent of activation led to a decrease in the fraction of molybdenum accessible to carbon monoxide. Photoelectron spectra for the MO-CO zeolite shows no Cls peak with BE near 288 eV, due to C-O bonds indicating, that the molybdenum precursor, MO (CO),, is fully converted to the oxide by calcination at 723 K [5,7]. Catalysts prepared by this method exhibit the characteristics of poorly dispersed molybdenum, in which the molybdenum oxide phases are located in the external surfaces. The use of MO ( CO)6 was clearly ineffective in depositing molybdenum in the zeolite structure of USY although Abdo and Howe [ 71 using the same precursor, located molybdenum cations in the zeolite supercages of H-Y zeolites which subsequently migrated into /I cages or D6R sites at higher activation temperatures. The detection of the MOO, phase by XRD and the relatively intense absorption bands due to nitric oxide, despite the lowest molybdenum content of the series (Table 1 ), indicate the presence of particulate molybdenum oxide phases. The absence of a significant reduction in water sorption capacity relative to USY would indicate that these molybdenum phases are externally located (Table 4). Such a location would be consistent with the highest measured Mo/Si ratio in the series (Table 1) . As discussed previously, a low integrated intensity at 900 cm-’ suggests the low abundance of the dispersed tetrahedral phase. This is also consistent with the highest value of proton acidity of the molybdenum loaded zeolites (Fig. 4 ), indicating an initially poor interaction between hydroxyl groups and the hexacarbonyl precursor. This contrasts for example with the complete dehydroxylation of HY zeolite following adsorption of MO (CO), and decomposition at 773 K [ 71. As USY contains a higher proportion of strong acid sites than HY, ultrastable zeolite may be expected to show a more facile oxidation of the Moo complex to Mo6+ by oxidation through its hydroxyl groups [ 5,7]. The poor dispersion of molybdenum formed using the MOM precursor in this study compared with previous reports [ 5,7] may be attributed to the use of a Mo( CO),-benzene solution, either resulting from the failure of the complex to enter the inner pore structure in the presence of benzene or due to its uncompetitive adsorption in the presence of benzene. XPS analysis of the MO-Cl sample revealed a trace of chlorine following calcination at 773 K. Dai and Lunsford [4] detected no chlorine for molybdenum exchanged Y and USY-zeolites but found retained chloride in samples of MoCl, supported on NaY zeolite after calcination at 673 K. This retention of Cl, in addition to the high level of remaining OH groups as determined by adsorption of pyridine (Fig. 4), would indicate hydrolysis of the precursor by water during preparation in air occurred during formation of the molybdenum oxide phase. Distinct differences in the nature of the molybdenum phases for
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54
53
MO-Cl and MoNa-Cl (Table 1), where the latter is essentially in the base form, would indicate that a degree of grafting between hydroxyls of the zeolite and the chloride precursor (in MoCl, or MoOCl, forms) had taken place for MoCl. In addition to chemical differences between USY and NaY forms, structural differences may also influence the molybdenum distribution in the two zeolites. The ca. 6% loading for both samples without structural losses is consistent with the value reported for Mo-mordenite prepared using the same precursor [ 21. However, the Mo/Si ratio of 0.149 for MoNa-Cl compared to 0.446 for MO-Cl (Table 1) , coupled with the lower nitrogen adsorption volume and the lower maximum water adsorption volume for the former, would indicate a lower internal molybdenum loading for the USY form (considering the similar molybdenum loading in both samples). The location of molybdenum ions in the zeolite cavities by use of MoCl, is consistent with previous results for MoUSY [4]. CONCLUSIONS
The distribution and nature of molybdenum species in USY zeolites is strongly influenced by the precursor and the method of preparation. The combination of experimental techniques used here indicates that preparation using MO ( CO)6 leads to an external loading of molybdenum oxide, whereas the other precursors locate a higher quantity in the internal cavities. Long periods of calcination at low constant pressure and elevated temperature increases the quantity of the latter by formation of mobile oxide or hydroxide species which migrate to the internal cages producing highly dispersed tetrahedral molybdenum species. ACKNOWLEDGEMENT
This work was supported by EC-Research Programme JOULE (Contract 0049). We thank the Royal Society for a European Science Exchange grant (J.A.A. ), and Dr. P. Magnus of CONTEKA, B.V., Surte (Sweden) for providing the USY zeolite.
REFERENCES 1 2 3 4 5
C.V. Caceres, J.L.G. Fierro, J. Lazaro, A. Lopez Agudo and J. Soria, J. Catal., 122 (1990) 113. J.R. Johns and R.F. Howe, Zeolites, 5 (1985) 251. H.K. Beyer, H.G. Karge and G. Borbely, Zeolites, 8 (1988) 79. P.S. Dai and J.H. Lunsford, J. Catal., 64 (1980) 173. M.B. Ward and J.H. Lunsford, in D.A. Olson and A. Bisio (Editors), Proceedings of 6th International Conference on Zeolites, 1983, Butterworths, Guildford, 1984, p. 405.
54 6
J.A. Anderson
et al./Appl. Catal. A 99 (1993) 37-54
P. Gallezot, G. Coudurier, M. Primet and B. Imelik, Am. Chem. Sot. Symp. Ser., 40 (1977) 144. 7 S. Abdo and R.F. Howe, J. Phys. Chem., 87 (1983) 1713. 8 P.Kovacheva, N.Davidova and DShopov, Zeolites, 3 (1983) 92. 9 I. Balakrishnan, S.G. Hedge, B.S. Rao, S.B. Kulkarni and P. Ratnasamy, in H.F. Barry and P.C.H. Mitchell (Editors), Proceedings of the 4th International Conference of the Chemistry and uses of Molybdenum, Climax Molybdenum Co., London, 1982, p. 331. 10 R. Cid, F.J. Gil Llambias, J.L.G. Fierro, A. Ldpez Agudo and J. Villaseiior, J. Catal., 89 (1984) 478. 11 J.L.G. Fierro, J.C. Conesa and A. Lopez Agudo, J. Catal., 108 (1987) 334. 12 A. Lopez Agudo, R. Cid, F. Orellana and J.L.G. Fierro, Polyhedron, 5 (1986) 187. 13 J.A. Anderson, J.L.G. Fierro, B. Pawelec, P.L. Arias and J.F. Cambra, Appl. Catal. A, 99 (1993) 55. 14 M.A.Baiiares, B. Pawelec and J.L.G. Fierro, Zeolites, 12 (1992) 882. 15 P.C.H. Mitchell and F. Trifirb, J. Chem Sot. A, 3183 (1970). 16 W. Newton and J. McDonald, in H.F. Barry and P.C.H. Mitchell (Editors), Proceedings of the 2nd International Conference of the Chemistry and uses of Molybdenum, Climax Molybdenum Co., London, 1976, p. 25. 17 J.B. Peri, J. Phys. Chem.,86 (1982) 1615. 18 A. Kazusaka and R.F. Howe, J. Catal., 63 (1980) 447. 19 L. Portela, P. Grange and B. Delmon, in L. Guczi, F. Solymosi and P. Tetenyi (Editors), Proceedings 10th International Congress on Catalysis, Budapest, 1992 (Studies in Surface Science and Catalysis, Vol. 75), part A, Elsevier, Amsterdam, 1993, p. 559. 20 E.P. Parry, J. Catal., 2 (1963) 371. 21 H.C. Yao and W.G. Rothschild, in H.F. Barry and P.C.H. Mitchell (Editors), Proceedings of the 4th International Conference of the Chemistry and uses of Molybdenum, Climax Molybdenum Co., London, 1982, p. 31. 22 KS. Chung and F.E. Massoth, J. Catal., 64 (1980) 320. 23 B. Pawelec, unpublished results. 24 P. Kovacheheva, N. Davidov and J. Novakova, Zeolites, 11 (1991) 54. 25 M. Cornac, A. Janin and J.C. Lavalley, Infrared Phys., (1984) 143. 26 F.A. Cotton and A.M. Wing, Inorg. Chem., 6 (1965) 867. 27 H.J. Jung, J.L. Schmitt and H. Ando, in H.F. Barry and P.C.H. Mitchell (Editors), Proceedings of the 4th International Conference of the Chemistry and uses of Molybdenum, Climax Molybdenum Co., London, 1982, p. 246.
Applied Catalysis A: General, 99 (1993) 37-54 Elsevier Science Publishers APCAT
B.V., Amsterdam
A2507
MO-USY zeolites for hydrodesulphurization. I. Structure and distribution of molybdenum oxide phase J.A. Anderson, B. Pawelec and J.L.G. Fierro Znstituto de Catcilisisy Petroleoquimica, C.S.Z.C., Campus UAM, Cantoblanco, 28049 Madrid (Spain) (Received 19 January 1993)
Abstract Ultrastable (USHY) zeolite has been used as a support material for a series of molybdenum catalysts prepared using various precursors and activation procedures. Studies using atomic absorption spectroscopy, X-ray photoelectron spectroscopy, Fourier transform IRof the mid-infrared range and of adsorbed nitric oxide and pyridine, and sorption capacity of water indicate that the distribution of molybdena in the zeolite is strongly influenced by the preparation procedures. Catalysts calcined by a non-conventional procedure, involving decomposition of the precursor by heating under vacuum over extended periods of time, show a migration of the molybdena from the external surface into the lattice cavities. Keywords:
catalyst characterization
(FT-IR,
XPS, XRD);
molybdena-USY
zeolites
INTRODUCTION
Supported molybdenum-based catalysts represent an important group of catalysts used in hydrotreating processes. The structure of the active phase in such catalysts is of paramount importance in determining catalytic activity. In this respect, the method of active phase incorporation and the nature of the support have been shown to play an important role in the kind of molybdena phase produced using conventional supports [ 11. In the case of zeolite supports with uniform pores in the framework structure, one may expect to form molybdenum species which are higher dispersed and with a more clearly defined structure than those species obtained over amorphous silica or alumina supports. Amongst these zeolites, ultra&able Y is of considerable interest due to its high thermal and chemical stability. However, in addition to the low ionexchange capacity of USY zeolite, conventional ion-exchange procedures are inapplicable in the case of molybdenum due to the lack of simple cationic forms Correspondence to: Dr. J.A. Anderson, Instituto de Cat&lisis y Petroleoquimica, UAM, Cantoblanco,
0926*860X/93/$06.00
28049 Madrid, Spain. Tel. (+34-1)5854773,
0 1993 Elsevier Science Publishers
B.V.
C.S.I.C., Campus
fax. (+34-1)5854760.
All rights reserved.
38
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54
of molybdenum which exist under conditions in which exchange would be possible. Various preparation methods for molybdenum loaded zeolites have been described and the resultant materials characterised in the forms MoHY, MoNaY and MOUSY. These methods include solid-state ion exchange using MoCl, [ 241, adsorption-decomposition of MO (CO), [ 5-71, and conventional impregnation with aqueous solutions of ammonium heptamolybdate (AHM) [8-lo]. Although the latter impregnation method produces essentially an external surface loading of molybdenum due to the failure of oxoanionic or neutral complexes to penetrate the zeolite cavities in the presence of water, thermal decomposition of these species under constant low vapour pressures of water produces a redispersion of molybdenum due to solid-state exchange of MoO,,(OH), within the zeolite pores [ 11,121. This procedure has been successfully applied in the cases of NaY and NH,NaY zeolite catalysts [ 11,121. In this paper, characterization data are reported for a series of MOUSY catalysts in which molybdenum has been incorporated using the above procedures. In the accompanying paper [ 131, characterization data for the sulphided catalysts along with activity and selectivity data for the hydrodesulphurisation of thiophene will be presented. EXPERIMENTAL
Catalyst preparation Four molybdena-loaded ultrastable Y (USY) zeolite catalysts were prepared using various procedures and different precursors. All the catalysts were prepared from the original ultrastable HY zeolite, kindly supplied by CONTECA, B.V., Surte (Sweden). Zeolite characteristics were: (a) USY (Conteca); Si0.J A1203 mole ratio 5.6, Na,O content 0.14 wt.-%, unit cell 2.454 nm and (b) NaY (LZ-Y52 Union Carbide); SiO,/Al,O, mole ratio 4.8, Na,O/Al,O, - 0.97. Catalyst MO-Cl was prepared by solid-solid ion exchange by mixing the starting USY zeolite with MoCl, [2-41. The USHY zeolite was firstly dehydrated in flowing helium at 493 K for 2 h to minimise hydrolysis of MoCl, by molecular water held by the zeolite. The dried zeolite was immediately mixed with anhydrous MoCl, and ground under air using a mortar and pestle. The resulting mixture was subsequently introduced in a flow reactor and heated in flowing helium at 673 K for 6 h. After this pretreatment, the catalyst was dried at 383 K and calcined at 773 K in air. Catalyst MO-CO was prepared by wet impregnation of USHY zeolite with the neutral MO ( CO)6 complex [5-71. This impregnation was carried out with a Mo(CO),-benzene solution whose volume was selected to achieve a final MOO, content of 6.0 wt.-%. The adsorbed complex was decomposed under vac-
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54
39
uum at 333 K and then dried in air at 383 K for 1 h. Calcination was conducted at 723 K for 1 h in a glass vessel. Two catalysts were prepared by conventional aqueous impregnation of an USY zeolite with ( NH1)6M07024-4H20 (AHM) [ 8-10 1, followed by removal of water in a rotary evaporator. These catalysts will be referred to as 4Mo-A and 8Mo-A, indicating low and high molybdenum contents, respectively. The impregnates were dried in vacuum for 8 h and later in air at 383 K. Calcination was achieved at 773 K at a low and constant pressure of water (4. 10m2Torr, 1 Torr= 133.3 Pa) and extended over a period of time of 800 h [ 11,121. For comparative purpose an MoOJNaY zeolite catalyst (MoNa-Cl) was prepared according to the same procedure used for the MO-Cl sample.
Catalyst churacterization Atomic absorption spectroscopy (AAS) Molybdenum contents were determined using a Perkin-Elmer 3030 atomic absorption spectrometer. Samples were prepared for analysis by dissolving in a mixture of HF, HCl and HNO, before heating in a microwave oven at a maximum power of 650 W.
Adsorption capacities Adsorption of water in the molybdenum-loading zeolites was measured using a Cahn-2000 vacuum microbalance. All samples were thoroughly outgassed at 623 K and were contacted with the adsorbate at 298 K at relative pressures (Pl%) between 0 and 1. The curves were constructed in graduated steps by increasing the pressure in small successive increments by evaporating distilled water from an ampoule. The equilibrium pressure was measured with a baratron MKS capacitance pressure transducer connected to the same vacuum line. From the isotherms of water adsorption, the maximum volume of the zeolites accessible to water ( IV,,) was calculated from the ordinate at the origin of the line in the linear form logW= f [ (TlogP/P,,)‘] as defined by the Dubinin-Radusehkevitch model. Nitrogen adsorption isotherms were measured at 78 K using a Micromerics Digisorb 2600 on samples which were previously outgassed at 623 K.
X-ray diffraction (XBD) X-ray powder diffractograms of the catalysts were recorded on a Philips PW-1140 diffractometer using Cu Kcr radiation and a nickel filter. Power diffractograms of MO-USY and MoNaY zeolites samples were recorded over a range of 28 values from 5 to 40’.
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54
Infrared spectroscopy Mid-infrared spectra in the range 1200-400 cm-’ were obtained by pressing wafers of 1% MO zeolite in KBr. For FT-IR studies of adsorbed nitric oxide, catalysts in the form of self-supporting wafers of about lo-12 mg/cm2 thickness were reduced in a hydrogen flow at 773 K for 2 h in a portable infrared cell equipped with greaseless stopcocks and KBr windows. After reduction the catalysts were outgassed in dynamic vacuum for 0.5 h at the same temperature and then cooled to room temperature. Catalysts were subsequently exposed to 30 Torr NO and the IR spectra of chemisorbed nitric oxide recorded using a Nicolet ZDX spectrophotometer at a resolution of 4 cm-‘. Spectra of adsorbed nitric oxide were obtained by subtraction of a background spectrum prior to nitric oxide exposure in order to eliminate an overtone band of the zeolite lattice which appears in this region. X-ray photoelectron spectroscopy (XPS) X-ray measurements were performed with a commercial XPS spectrometer (Fisons escalab 200R) equipped with a Mg Ka X-ray source (h y = 1253.6 eV) and a hemispherical electron analyzer. The X-ray source was operated at 12 kV and 10 mA. The samples were pressed into small stainless steel cylinders and mounted onto a manipulator which allowed the transfer from the preparation chamber into the spectrometer. The samples were pumped out to 10M5 Torr before they were moved into the analysis chamber. The residual pressure in this ion-pumped chamber was maintained below 7*10-’ Torr during data acquisition. Each spectral region of the photoelectrons of interest was scanned several times to obtain good signal-to-noise ratios. The Cls peak at a binding energy of 284.9 eV was taken as an internal standard. RESULTS
AAS The chemical composition of the calcined MO-USY zeolites is presented in Table 1.The extent of molybdenum-exchange was rather low for catalyst MoCO, considering that the volume of solution of MO ( CO)6 was chosen to give a MOO, content of 6.0 wt.-%. For catalysts 4 and BMo-A, prepared by conventional aqueous impregnation of an USY zeolite with AHM and calcined by an isobaric thermal decomposition procedure, MOO, contents obtained were close to those calculated theoretically.
41
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54 TABLE 1 Chemical composition, infrared and XPS data for Mo-zeolite catalysts Catalyst
wt.-%
1730+ 1820 (cm
8Mo-A MO-Cl MoNa-Cl 4Mo-A MO-CO
8.6 6.3 5.4 3.9 3.1
-1
10.62 1.53 1.12 3.08 1.37
10
9201460 cm-‘/cm-’
MO 3dS,zb (eV)
0.162 0.104 0.006 0.081 0.059
232.3 232.4 232.3 232.8 232.6
IMcJzSi
0.526 0.446 0.149 0.174 0.462
“Integrated absorbance measured by nitric oxide adsorption at room temperature on Hz-reduced catalysts. batheaccuracy of each measurement is within 0.2 eV. ‘Intensity areas of the MO 3d and Si 2p peaks normalized for the number of scans.
,
20
35
28 angle (“) Fig. 1. XRD patterns for (a) USY, (b) 4Mo-A, (c) MO-CO, (d) MO-Cl, and (e) 8Mo-A.
X-ray diffraction The diffraction patterns of the catalysts and the USY zeolite (Fig. 1)showed peaks at d-spacings of 0.5644, 0.4733, 0.4348, 0.3750, 0.3285 and 0.2839 nm
42
J.A. Anderson
et al./Appl. Catal. A 99 (1993) 37-54
which are typical of USY zeolites. The crystallinity changes were estimated for all zeolite catalysts by comparing the three peaks at 0.5644, 0.3750 and 0.2839 nm with those of the parent USY zeolite. A slight decrease of ca. 10% was observed which can be explained in terms of the dilution effect of the molybdenum oxide added to the base zeolite [ 51. These results differ markedly from MoNa-Y catalysts which show larger crystallinity losses produced by calcination treatments [ 121. The MOO, phase, whose principal reflection for the orthorhombic phase occurs at a d-spacing of 0.345 nm (ASTM 21-569), was observed only for 8Mo-A and MO-CO zeolites, despite the fact that these samples contained the highest and lowest molybdenum contents, respectively (Table 1). Infrared spectroscopy Mid IR range, 1200-400 cm-’ Spectra of molybdenum loaded USY zeolites in KBr exhibited bands at 1200, 1070, 835,605, 503 and 465 cm -’ due to framework vibrations of the zeolite [4,7,14]. The presence of these bands in spectra for all the catalysts and the absence of significant intensity losses confirms that the crystallinity was retained for calcination temperatures between 723 and 773 K. Consistent with previous studies of MoY zeolites [ 4,6,7,14], an additional maximum at ca. 900 cm-l, not present for the USY zeolite in the absence of molybdenum, was detected for all samples. MO-O vibrations may be expected to give IR active bands in the range 800-1000 cm-’ [ 61. If this band is indicative of Moos, then this phase must have been present for all samples despite the detection by XRD (Fig. 1) in only 8Mo-A and MO-CO samples since the band at ca. 900 cm-’ was present for all catalysts. The integrated absorbance of the IR band at ca. 900 cm-’ (as a ratio using the structure insensitive band at 465 cm-’ as an internal standard) is shown in Table 1 for the molybdenum-loaded zeolite catalysts. On comparing these values with the Moos contents, it is clear that an increase in loading does not give an increase in intensity despite several reports to the contrary [4,14 1. The anomaly of sample MO-CO, which has a crystalline MoOB phase detected by XRD although having the lowest molybdenum content and the second lowest band intensity at ca 900 cm-’ (Table 1 ), would indicate that the band at ca. 900 cm-l is due to some surface MO-O species. This is consistent with the perturbation of a similar maximum in the presence of ammonia [ 71. It is of interest that the band maximum appears at ca. 895 cm-’ for samples MO-Cl and MO-CO but at ca. 920 cm-l with an unresolved lower frequency maximum for samples MO-A. Abdo and Howe [ 71 report the presence of resolved maxima at 900, 878 and 850 cm-’ following activation at 773 K of Mo(CO), in Y zeolite. Previous reports indicate that the frequency of the Mo=O vibration in this region depends on the coordination number and the number of terminal oxygen atoms on the molybde-
J.A. Anderson et al. jApp1. Catal. A 99 (1993) 37-54
43
num (VI) [ 151. Alternatively, a decrease in frequency in the Mo=O band may implicate an increase in the oxidation state of molybdenum [ 161. Fig. 2 shows the dependence of the intensity of the combined band envelope at ca. 900 cm-’ on the time of calcination under vacuum for catalysts 4Mo-A and 8Mo-A. A progressive increase in this intensity occurs for sample 4Mo-A during the course of the calcination treatment, whereas for 8Mo-A the growth is initially more dramatic but declines after extended periods at 773 K. Nitric oxide adsorption Two absorption bands at ca. 1730 and 1820 cm-’ were observed for nitric oxide adsorbed on all reduced samples as shown in Fig. 3A and B for 4Mo-A and 8Mo-A examples. These bands may be attributed to paired nitric oxide molecules held as dinitrosyls adsorbed at partially reduced, probably Mo4+ sites [ 17,181. This doublet appears at higher wavenumbers than for conventional molybdena-alumina catalysts [ 181, as in the case of dinitrosyl species formed on other metal oxide-loaded Y zeolites. An indication of the relative proportion of coo&natively unsaturated (CUS) reduced molybdenum sites for each catalyst was obtained by measuring the total integrated absorbance at ca. 1730 and 1820 cm- ’ (Table 1). The order of molybdena exposure in the various catalysts follows the trend: 8Mo-A > 4MoA > MO-Cl > MO-CO > MoNa-Cl. Spectra obtained for reduced samples of 8MoA and 4Mo-A as a function of calcination time at 773 K are shown in Fig. 3
3,5-
,c
-0906
/ / 3,0-
-0,07
2,6-
-0,06'2
z is 0 -0,os
w-
- 630
l,6-/
fl
-0,04
B 1,0I0,03 10 0
Calcination
time
(days)
Calcinatlon
20
time
30
40
(days)
Fig. 2. Dependence of the absorbance ratio 9001465 cm-’ and on the combined integrated intensities of nitric oxide absorption bands at 1820 and 1730 cm-’ on the calcination time for (A) 8Mo-A and (B) 4Mo-A.
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54
44
1860
1760
1660
1860
Wavenumber
(cm-‘)
1760
1660
Fig. 3. IR spectra of adsorbed nitric oxide at room temperature on H,-prereduced MO-loaded zeolites: (A) 4Mo-A; (B) 8Mo-A for calcination times of (a) 1, (b) 5, (c) 15, (d) 23 and (e) 33 days. TABLE 2 IR and XPS data for 4Mo-A catalyst calcined under vacuum for different periods of time
t
MO3d.w
(days)
(ev)
1 5 15 23 33
232.7 232.6 232.8 232.7 232.8
blol&.P
A
1730 -1
(cm
0.200 0.153 0.158 0.190 0.174
)
b
0.54 1.28 1.56 1.61 1.74
DSurfaceatomic ratio determined from XPS spectra. bInt.ergatedabsorbance measured by nitric oxide adsorption at room temperature on Hz-reduced catalysts.
with the integrated band intensities collected in Tables 2 and 3. In Fig. 2, the changes in the combined intensities at 1730 and 1820 cm-’ as a function of calcination time may be compared with the change in intensity of the band at ca 900 cm-’ due to MO-O vibrations. Sample 8Mo-A displays a decrease in the
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54 TABLE 3 IR and XPS data for SMo-A catalyst calcined under vacuum for different periods of time
t
MO W/z (days) (eV)
h0lh.i”
A 1730 cm-l’
1 5 15 23 33
0.689 0.447 0.570 0.442 0.526
1.77 5.35 6.90 11.34 6.5
232.4 232.4 232.3 232.6 232.3
“Surface atomic ratio determined from XPS spectra. bIntergated absorbance measured by nitric oxide adsorption at room temperature on Hz-reduced catalysts.
total absorption after extended periods of time whereas the increase for sample 4Mo-A is less dramatic but continual. In addition to changes in the overall intensities as a function of calcination time, spectra for sample 8Mo-A clearly show changes in the relative intensities of bands due to asymmetric and symmetric stretching vibrations and a shift to lower frequencies for the latter maximum, which are not displayed by the 4Mo-A sample. This shift has been interpreted as indicative of an increase in the degree of unsaturation [ 191. Pyridine adsorption As hydroxyl groups, either as a source of protons or in condensation processes, react with MoCl, (or MoOCl,) [ 2-41, MO ( CO)6 [ 5-71 or with molybdate anions derived from the calcined precursors [ 121, experiments were carried out involving the adsorption of pyridine to determine the extent to which proton acidity was suppressed following incorporation of molybdenum. Brensted acid sites are typically identified by a band at ca. 1540 cm-’ corresponding to pyridinium ions and Lewis acid sites by a band near 1440 cm-’ due to coordinated pyridine molecules [ 201. From the spectra in Fig. 4 obtained by adsorption of pyridine at 298 K followed by outgassing at 393 K, an estimation of the ratio of Brensted to Lewis acidity was calculated from the ratio of integrated band intensities at ca. 1540 and ca. 1440 cm-‘. In decreasing order these were: USY > MO-CO > MO-Cl z 4Mo-A x 8Mo-A > MoNa-Cl. The detection of Brensted acidity for the Na-Y zeolite (Fig. 4) is surprising although sodium doped alumina samples containing an excess of these cations also exhibit proton acidity after calcination at 573 K [ 201. The MoNa-Cl sample exhibited a band at 1444 cm- ’ due to adsorbed pyridine which was detected at 1452 cm-’ for the MO-USY samples indicating weaker Lewis acid centres in the presence of sodium. This is consistent with the shift to lower frequencies disfilayed by pyridine adsorbed on sodium doped A1203 compared with sodium free A1203 [ 201.
46
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54
1700
1600
1500
1400
Wavenumber(cm-‘) Fig. 4. IR spectra of pyridine adsorbed at 298 K on (a) MoNa-Cl, (b) USY, (c) MO-Cl, (d) MOCO, (e) 4Mo-A, (f) 8Mo-A.
X-ray photoelectron spectra XP spectra for the calcined MO-USY zeolites and MoNa-Cl catalyst are shown in Fig. 5 and the Mo3d binding energies summarised in Table 1. The binding energy of the most intense Mo3d peak at ca. 232.5 eV was essentially the same for all catalysts and is consistent with values expected for fully oxidized molybdenum [ 4,111. The Mo3d peaks for the MoNa-Cl sample (Fig. 5e) shows a third component towards lower binding energies. As additional peaks with the same shifts were observed for A12p, Si2p and Nals using the same sample, this may be attributed to charging effects. The Mo3d peak intensities relative to silicon are given in Table 1 for the series of samples while the surface atomic ratios of Mo/Si for the 4Mo-A and 8Mo-A catalysts calcined under vacuum for different periods of time are listed in Tables 2 and 3, respectively. Both samples show lower Mo/Si ratios after prolonged calcination periods compared to values after 1 day calcination. Fig. 6 shows the relationship between XPS Mo/Si ratios and the overall (1730 and 1820 cm-‘) absorbance for the series of Mo-USY zeolites and MoNaY sample. Points for MO-Cl, MoNa-Cl and MO-CO lie on an straight line show-
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54 MO 3d
47
512
1
I
I
240
235
230
BE (eV) Fig. 5. Mo3d core level spectra for (a) 4Mo-A, (b) 8Mo-A, (c) MO-Cl, (d) MO-CO and (e) MoNaCl.
ing a correlation between the two factors [ 191. However, the MO-A samples, which adsorbed higher quantities of nitric oxide do not show the same correlation, with sample 8Mo-A in particular, far from the line indicating a considerable fraction of molybdenum was accessible to nitric oxide but not detected by KPS. Water acEsorption Water adsorption isotherms at 298 K for the various molybdenum-loaded zeolkes and the parent USY zeolite show a rapid increase in water adsorption at P/P0 below 0.2, followed by a slow increase up to P/P,, of 0.9. As these
48
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54
2
Absorbance
(au x cm.‘)
Fig. 6. Relationship between the XPS Mo/Si ratios and the overall (1820 + 1730 cm-’ ) integrated absorbance of bands due to nitric oxide for 4Mo-A (0 ), MoNa-Cl ( A ), MO-Cl (m), MO-CO ( •I1 and 8Mo-A xeolites ( 0 ) . TABLE 4 Water adsorption at 298 K (P/P,,=O.4) USY zeolites
and nitrogen adsorption at 78 K (P/P0=0.2)
Zeolite
W wetel (w/g,,)
Woo (cm3/h)
vNitmgen
4Mo-A 8Mo-A MO-Cl MO-CO MoNa-Cl USY NaY
238 205.7 245.5 315 213.3 355 331.8
0.257 0.219 0.256 0.331 0.211 0.372 0.331
125.6 124.9 133.0 155.9 120.5 134.6 190.0
for Mo-
(cm”/g,,J
“W, is the maximum volume of the zeolite accessible to water.
isotherms indicate the amount of liquid water condensed in the zeolites pores and cavities, a comparison of the extent of water adsorbed can be made at a fixed P/P0 value of 0.4 (Table 4). From these results of water sorption capacity, an indication of the location of Moo3 in the different molybdenum-loaded zeolites may be obtained. Similar values of sorption capacity were obtained for the MO-CO zeolite and unloaded USY and Nay. The slight decrease for MoCO may be a diluent effect of MOO,. The high sorption capacity of the MO-CO sample is consistent with the external location of a poorly dispersed MOO, phase in the calcined sample. The differences are much more notable for the other samples. The 4Mo-A, MO-Cl and 8Mo-A samples gave rather low values. As both 4Mo-A and 8Mo-A zeolites were prepared by impregnation with heptamolybdate anions which cannot enter the zeolite pores, the micropore vol-
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54
49
ume, and water sorption capacity, would be expected to be similar to the parent USY zeolite. As indicated in previous studies [ 2,111, nitrogen isotherms for zeolites cannot be fitted to the BET equation, showing instead type 1 behaviour. However adsorption capacities at P/P,, = 0.2 can be compared with the unloaded zeolite to yield information regarding the internal location of molybdena species [ 21. The results for nitrogen adsorption (Table 4) are in general agreement with the data for water adsorption with samples 8Mo-A, 4Mo-A and MO-Cl showing the highest losses in volume compared with the unloaded zeolite, and MO-CO showing a relatively smaller loss. The maximum loss in capacity of 32% (~MoA) is identical to a value reported for 6% MO in mordenite after activation at 723~K [2]. DISCUSSION
The combination of mid infrared and XRD measurements indicates that the crystallinity of the MOUSY zeolites is maintained even when the precursors are activated at temperatures of 723 to 773 K, unlike MO-Y zeolite where appreciable losses in crystallinity occur in this temperature range [ 4,7,12]. From Fig. 6, where the Mo/Si ratio from XPS has been plotted against the integrated intensity of the IR bands of adsorbed nitric oxide, the MO-A samples and in particular 8Mo-A appear anomalous in the series. The values for the other samples lie close to a straight line indicating correlation between the two aforementioned parameters and consistent with similar measurements for Mo/A1203 [ 191. The considerable deviation from the line of data for BMo-A, and to a lesser extent for 4Mo-A, would suggest the presence of a molybdenum phase, accessible to nitric oxide molecules, but located within the inner zeolite structure and hence not detected by XPS. This does not mean that the other samples contained only externally located molybdenum phases: only that the rel: at&e proportion was higher for the MO-A samples. The extent of molybdenum location within the internal zeolite structure may be obtained from the occlusion of water and nitrogen adsorption relative to the molybdenum free USY zeohte (Table 4). These data clearly indicate the relatively large volumes in the :inner zeolite structure penetrated by molybdenum phases for 8Mo-A and MoNa-Cl. As 8Mo-A and 4Mo-A were prepared using heptamolybdate anions which are not expected to enter the zeolite pores in the presence of water [8121 this suggests a transfer of molybdenum occurs during the long calcination procedure adopted. This would be consistent with the variation in the number of nitric oxide adsorption centres (Tables 2, 3, Fig. 2) as a function of the number of days of calcination and also the drop in Mo/Si ratio following the first day of calcination. The redispersion mechanism of molybdena in zeolites has received some attention [11,12,14], and leads to the migration of MOO, from the external
50
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54
zeolite surface into the lattice cavities during calcination. Such a process may involve vaporization of MOO, and/or MOO, (OH) 2, the latter being formed according to the equilibrium: MoO,(s)+H,O(v)~MoO,(OH),(v)
(1)
Although the vapour pressure of MOO, and Moz (OH), are low at 773 K [lo], the extended calcination periods and the short distances (the same order as the zeolite grains) across which the diffusion process needs to be operative, allows an efficient molybdenum redistribution. The molybdena concentration will tend towards uniformity throughout the zeolite structure, except inside the smaller cavities where steric hindrance effects may be important. Assuming a diameter of 0.47 nm (equivalent to 0.17 nm2 cross section) for adsorbed Moos or Moo2 (OH),, entrance into the sodalite units and hexagonal prisms would be diffusion controlled at high temperatures. The resultant molybdenum species would be highly dispersed and located on a large variety of sites producing a wide heterogeneity in molybdenum-zeolite interactions. The variation in the intensity ratio of the bands due to the asymmetric/symmetric stretching vibrations of adsorbed nitric oxide indicating a change in dipole angle, has been related to the different topological characteristics of the surface structure of the molybdenum phases [ 211. Both samples after 1 day calcination show nitric oxide adsorbed in a form with a dipole angle (Q) of ca. 69”. However whereas the value for 4Mo-A remains constant thereafter at ca. 100” the values for 8Mo-A vary from 100 to 132”, indicating a continual change in the nature of the CUS molybdenum sites. A redispersion of molybdenum species for 8 and 4Mo-A samples, would be consistent with the changes in band intensity at ca. 900 cm-’ as a function of calcination time (Fig. 2). The 8Mo-A and 4Mo-A samples also exhibited initial increases in the quantity of CUS reduced MO*+ sites as monitored by the intensity of the vN0 band (Fig. 2), indicating a possible connection between these measurements. This apparent correlation need not indicate that these parameters (reduced CUS nitric oxide adsorption sites and band intensity at ca. 900 cm-‘) are related to the same molybdenum species. This is evident from the different numerical order of magnitude for the two parameters given in Table 1 for the complete series of samples. A redispersion (increase in intensity at ca. 900 cm-‘) may produce adsorbed molybdenum species, stable to reduction in hydrogen at 773 K, and therefore incapable of nitric oxide adsorption. Such a species would be consistent with the non-reducible tetrahedral molybdenum species formed at low loadings on A1203 [ 221. In this respect, fully calcined 8 and 4Mo-A samples exhibited an intense maximum at ca. 230 nm in the diffuse reflectance UV spectra, consistent with the presence of molybdenum in tetrahedral coordination [lo] with only a very weak maximum at 330 nm for the 8 MO-A sample [ 231. The presence of the latter maximum, characteristic of Moo3 [lo], is consistent with the detection of this phase by
51
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54
XRD (Fig. le). The formation of this tetrahedral species is consistent with the growth in band intensity at ca. 900 cm-‘, the development of the higher frequency maximum at 920 cm-’ for MO-A samples, and with previous assignments to MOO,‘- species in MoUSHY, MoNaY and MoY zeolites [4,10,24]. The higher frequency maximum compares with a band at 920 cm-’ observed at low molybdenum loadings on SiO, and attributed to pseudomolybdates [ 251. Daiiand Lunsford [ 41 attributed a similar band to species (I) in MOUSY zeolite,;whereas the development of the higher frequency band leads us to believe the growth in this region as a function of time (Fig. 2) to be due to formation of species (II), where z indicates an oxygen ion in the zeolite lattice. OH
o3\ 03
/
MoVI< 03
0 o\\ MoVI< oz OZ’
(11)
(1)
The tetrahedral cis-dioxo-molybdenum (VI) complexes have Mo=O frequencies of 876-913 and 909-943 cm-’ [26]. The formation of such species may be readily envisaged, following reaction of the mobile species produced in eqn. (1) with the surface.
MoO,(OH),
+ 2(OH)z
-
O\\ //O
+ 2H,O
(2)
oz~Mo~oz Despite the increase in quantity of these species formed as a function of Cal&nation time (Fig. 2), the unreducible nature of these MoV’ species in hydrogen at 773 K [ 10,171 makes these centres incapable of nitric oxide adsorption. The adsorption sites for nitric oxide are probably located on the larger polymolybdate structures or MOO, crystallites. Break up of these with the formation of mobile MoOz (OH )Z species would initially create additional exposed reducible MoV’ ions and subsequently an increase in the number of nitric oxide adsorption sites. The above conclusions are in line with the speculative proposal of Jung et al. [27], that only MOO, species, such as bulk MOO, crystalliteg, which are not directly bonded to the support may chemisorb nitric oxide. In a previous study [ 111, it was suggested that the bonding of the mobile MoOz (OH), type species to MOO, was weaker than to the zeolite surface so that after an efficient redispersion on the external surface, a higher temperature was required to detach these species and allow further diffusion into the bulk of the solid and the zeolite small cavities. The loss in nitric oxide adsorp-
52
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54
tion capacity (Fig. 2)) may be explained in a similar manner: not so much following an increase in temperature but due to the extended period of time allowing the mobile species to enter cavities inaccessible to nitric oxide. Abdo and Howe [ 71 report that for MoHY, an increase in the extent of activation led to a decrease in the fraction of molybdenum accessible to carbon monoxide. Photoelectron spectra for the MO-CO zeolite shows no Cls peak with BE near 288 eV, due to C-O bonds indicating, that the molybdenum precursor, MO (CO),, is fully converted to the oxide by calcination at 723 K [5,7]. Catalysts prepared by this method exhibit the characteristics of poorly dispersed molybdenum, in which the molybdenum oxide phases are located in the external surfaces. The use of MO ( CO)6 was clearly ineffective in depositing molybdenum in the zeolite structure of USY although Abdo and Howe [ 71 using the same precursor, located molybdenum cations in the zeolite supercages of H-Y zeolites which subsequently migrated into /I cages or D6R sites at higher activation temperatures. The detection of the MOO, phase by XRD and the relatively intense absorption bands due to nitric oxide, despite the lowest molybdenum content of the series (Table 1 ), indicate the presence of particulate molybdenum oxide phases. The absence of a significant reduction in water sorption capacity relative to USY would indicate that these molybdenum phases are externally located (Table 4). Such a location would be consistent with the highest measured Mo/Si ratio in the series (Table 1) . As discussed previously, a low integrated intensity at 900 cm-’ suggests the low abundance of the dispersed tetrahedral phase. This is also consistent with the highest value of proton acidity of the molybdenum loaded zeolites (Fig. 4 ), indicating an initially poor interaction between hydroxyl groups and the hexacarbonyl precursor. This contrasts for example with the complete dehydroxylation of HY zeolite following adsorption of MO (CO), and decomposition at 773 K [ 71. As USY contains a higher proportion of strong acid sites than HY, ultrastable zeolite may be expected to show a more facile oxidation of the Moo complex to Mo6+ by oxidation through its hydroxyl groups [ 5,7]. The poor dispersion of molybdenum formed using the MOM precursor in this study compared with previous reports [ 5,7] may be attributed to the use of a Mo( CO),-benzene solution, either resulting from the failure of the complex to enter the inner pore structure in the presence of benzene or due to its uncompetitive adsorption in the presence of benzene. XPS analysis of the MO-Cl sample revealed a trace of chlorine following calcination at 773 K. Dai and Lunsford [4] detected no chlorine for molybdenum exchanged Y and USY-zeolites but found retained chloride in samples of MoCl, supported on NaY zeolite after calcination at 673 K. This retention of Cl, in addition to the high level of remaining OH groups as determined by adsorption of pyridine (Fig. 4), would indicate hydrolysis of the precursor by water during preparation in air occurred during formation of the molybdenum oxide phase. Distinct differences in the nature of the molybdenum phases for
J.A. Anderson et al./Appl. Catal. A 99 (1993) 37-54
53
MO-Cl and MoNa-Cl (Table 1), where the latter is essentially in the base form, would indicate that a degree of grafting between hydroxyls of the zeolite and the chloride precursor (in MoCl, or MoOCl, forms) had taken place for MoCl. In addition to chemical differences between USY and NaY forms, structural differences may also influence the molybdenum distribution in the two zeolites. The ca. 6% loading for both samples without structural losses is consistent with the value reported for Mo-mordenite prepared using the same precursor [ 21. However, the Mo/Si ratio of 0.149 for MoNa-Cl compared to 0.446 for MO-Cl (Table 1) , coupled with the lower nitrogen adsorption volume and the lower maximum water adsorption volume for the former, would indicate a lower internal molybdenum loading for the USY form (considering the similar molybdenum loading in both samples). The location of molybdenum ions in the zeolite cavities by use of MoCl, is consistent with previous results for MoUSY [4]. CONCLUSIONS
The distribution and nature of molybdenum species in USY zeolites is strongly influenced by the precursor and the method of preparation. The combination of experimental techniques used here indicates that preparation using MO ( CO)6 leads to an external loading of molybdenum oxide, whereas the other precursors locate a higher quantity in the internal cavities. Long periods of calcination at low constant pressure and elevated temperature increases the quantity of the latter by formation of mobile oxide or hydroxide species which migrate to the internal cages producing highly dispersed tetrahedral molybdenum species. ACKNOWLEDGEMENT
This work was supported by EC-Research Programme JOULE (Contract 0049). We thank the Royal Society for a European Science Exchange grant (J.A.A. ), and Dr. P. Magnus of CONTEKA, B.V., Surte (Sweden) for providing the USY zeolite.
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